Fluorescent lamps are generally used to achieve the good luminous efficiency produced thereby. In addition, the long operating life of fluorescent lamps and the various colour tones enable the use of the lamps in a plurality of different applications.
Burning fluorescent lamps requires a supply coupling to supply the required ignition voltage to the lamp circuit and the supply voltage required during use. If the supply coupling is formed without active monitoring of the lamp circuit, the problem that arises is that when the fluorescent lamp or the supply circuit gets damaged, it is impossible to automatically switch off the supply transformer, but it continues to supply the lamp circuit with power until manually switched off. When a broken lamp circuit is fixed, i.e. typically replaced with an undamaged fluorescent lamp, the supply transformer has to be switched on again manually.
Finnish publication 107580B discloses a supply coupling of a lamp circuit, wherein the above problem is rectified by adding a separate direct-current supply to the supply coupling, the direct-supply supplying direct current through the cathodes of the fluorescent lamp and the secondary of the supply transformer to a separate indicator circuit, which is arranged to indicate the direct current generated by the direct-current supply and thus the working condition of the fluorescent lamp. The purpose of the indicator circuit is to transfer information about the working condition of the fluorescent lamp to electronics controlling the supply of the fluorescent lamp and to stop the supply of voltage should the lamp or the lamp circuit be damaged.
However, the problem with the solution of the publication is that direct current causes a small direct-current component in the alternating current passing through the fluorescent tube. This direct-current component subjects the electrodes of the fluorescent tube to polarization, i.e. one electrode transforms into a cathode and the other into an anode. This tends to cause uneven wear of the electrodes, whereby active substance is oxidized on the anode and reduced on the cathode. Consequently, the operating life of the electrode acting as the anode shortens.
Furthermore, the prior art separate indicator circuit is located at the potential of the secondary of the supply transformer, whereby the voltage or current message produced by the indicator circuit has to be transferred to the potential of the electronics controlling the coupling device, i.e. the primary of the supply transformer.